In a speech given at the Kennedy
Space Center last month, President Obama reaffirmed his administration's
decision to cancel Constellation, NASA's program to create new vehicles for
human flights to the moon and Mars. If implemented, this decision will
guarantee a decade of non-achievement by NASA's human spaceflight program, at a
cost of more than $100 billion.

Although we are known for
holding different opinions on the order and importance of specific objectives
in space, we are united in our concern over this move to turn away from the
Vision for Space Exploration (hereafter referred to as Vision). Vision gave
NASA's human spaceflight program a clear direction: to reach the moon and Mars.
Congressional authorization bills in 2005 (under Republican leadership) and
2008 (under Democratic leadership) endorsed this goal.

The agency created the
Constellation program to build the Ares 1 and Ares 5 launch vehicles, the Orion
spacecraft and other hardware needed to go to the moon and Mars. A timeline was
set, and objectives were articulated to achieve Vision's first major milestone
- a sustainable return to the moon by the end of the present decade to gain
knowledge, reacquire operational experience and use local resources to create
capabilities for our reach to Mars and beyond.

Vision had its roots in the
2003 report of the Columbia Accident Investigation Board, which asserted that
the goals associated with human spaceflight must be worthy of its costs and
risks. In canceling Constellation and Vision, the administration is proposing
to return NASA to its pre-Columbia template of operating on a "flexible
path" involving no commitment to any specific timeline, achievement or
objective. This new direction, coming just as the space-shuttle program is set
to end, threatens America's human spaceflight effort not merely with stagnation
but also with cancellation.

The new plan proposes to
contract with private companies to design and develop vehicles for human
flights to low Earth orbit (LEO) and the International Space Station. The
agency will research advanced technologies in the coming five years before
picking a heavy-lift rocket design. Human missions are next - to an asteroid in
15 years and to orbit Mars in 25 years. A human Mars landing supposedly will
occur afterward - sometime.

The idea of contracting with
the private sector for launch and transport to LEO is not new. This capability
was encouraged and started under Vision. The difference under the new direction
is the termination of any capability by the federal government of the United States
to send people into space.

For 50 years, America has
maintained this ability through an infrastructure of cutting-edge industrial
hardware, specialized facilities and a skilled work force. By adopting the new
program, we will lose - probably irretrievably - this space-faring
infrastructure and, most certainly, our highly trained, motivated and
experienced work force. It will be prohibitively expensive and difficult to
restart our manned program after five to 10 years of agency navel-gazing,
effectively signaling the end of America's manned space program and our
leadership in space.

NASA falters without
specific direction or a stated destination. The history of the agency is
replete with research projects disconnected from flight missions that produced
no real hardware or technology. Taking five years (or even one year) to
"study" the technologies of a heavy-lift rocket is not only pointless
- it is destructive. We currently possess all the knowledge, technology and
infrastructure necessary to build a heavy-lift launch vehicle.

In a logical and effective
space program, a mission is chosen, a plan for accomplishing the mission is
developed, the flight hardware needed to accomplish the plan is identified, and
technology is developed as needed to enable the flight hardware. The
administration claims it is setting daring goals - the asteroids and Mars - but
has posited them so far in the future that no real, focused work needs to be
done toward their achievement during this or the next presidential term.

Under Vision, we were working on the
development of real capabilities, including launch systems, spacecraft and
destinations with specific activities and capabilities at these places. If the
new path is adopted, we will have exchanged a mission-driven program for a
costly stagnation that will take us nowhere.

That is the choice before us.

Paul D. Spudis is
a planetary scientist, principal investigator of the Mini-SAR imaging radar on
the Chandrayaan-1 mission and author of "The Once and Future
Moon"(Smithsonian, 1996). Robert Zubrin is an astronautical engineer,
president of the Mars Society and author of "The Case for Mars: The Plan
to Settle the Red Planet and Why We Must"(Free Press, 1996).